Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. In recent years, applicability of secondary batteries has been realized as power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs). In the light of such trends, a great deal of research and study has been focused on secondary batteries which are capable of meeting various demands. Among other things, there has been an increased demand for lithium secondary batteries having high-energy density, high-discharge voltage and high-power output stability.
Particularly, lithium secondary batteries for use in EVs require not only a high-energy density and a capability to exert a large power output within a short period of time, but also a long-term lifespan of more than 10 years even under severe conditions in which high-current charge/discharge cycles are repeated within a short time, thus necessitating remarkably superior safety and long-term lifespan as compared to conventional small-size lithium secondary batteries.
Lithium ion secondary batteries that have been used in conventional small-size batteries generally employ a layered structure of lithium cobalt composite oxide as a cathode material and a graphite-based material as an anode material. However, the main constitutional element of the lithium cobalt composite oxide, cobalt, is very expensive and is not suitable for use in electric vehicles due to safety concerns. Therefore, as the cathode material of lithium ion batteries for EVs, a lithium manganese composite oxide having a spinel structure may be suitable which is made up of cheap and highly safe manganese.
However, the lithium manganese composite oxide, upon high-temperature and large-current charge/discharge, undergoes dissolution of manganese ions into an electrolyte due to the influence of the electrolyte, thus resulting in degradation of battery properties and performance. Thus, there is a need for measures to prevent such problems. In addition, the lithium manganese composite oxide has drawbacks such as a low capacity per unit weight, i.e., a low charge density, as compared to conventional lithium cobalt composite oxides or lithium nickel composite oxides. Thus, there is a limit to the charge density of the battery. Also, in order to enter practical use as the power source of EVs, designs of the battery to solve such disadvantages should be created together.
In order to alleviate the above-mentioned respective disadvantages, various studies and attempts have been made to fabricate electrodes using a mixed cathode active material. For example, Japanese Unexamined Patent Publication Nos. 2002-110253 and 2004-134245 disclose techniques utilizing a mixture of a lithium/manganese composite oxide, and a lithium/nickel/cobalt/manganese composite oxide and/or a lithium/nickel/cobalt/manganese composite oxide to enhance a recovery output. These conventional prior arts, however, still suffer from problems associated with a poor cycle life of the lithium manganese oxide and limited improvement of safety.
Meanwhile, Korean Patent No. 0458584 discloses a cathode active material composed of a nickel-based, large-diameter active material compound having an average particle diameter of 7 to 25 μm and a small-diameter active material compound having an average particle diameter of 2 to 6 μm (for example, LixMn2O4−zXz, wherein X is F, S or P, 0.90≦x≦1.1, and 0≦z≦0.5), in order to increase a battery capacity by improving a volumetric density of an electrode plate.
Further, in order to improve capacity characteristics, lifespan characteristics and high-rate discharge characteristics of batteries, Korean Patent No. 0570417 discloses a secondary battery using a spinel LiMn2O4 cathode as a cathode active material, Korean Unexamined Patent Publication No. 2002-0080448 discloses a secondary battery using a cathode active material containing a lithium manganese composite oxide, and Japanese Unexamined Patent Publication No. 2004-134245 discloses a secondary battery using a cathode active material containing a spinel lithium manganese composite oxide and a lithium transition metal composite oxide.
However, the construction of the secondary battery having a combination of desired levels of the lifespan and safety has not yet been proposed, despite the aforementioned conventional prior arts.